Customers of print jobs can require finishing steps for their jobs. These steps include, for example, folding printed or blank sheets, cutting sheets, trimming sheets to size and shape, cutting specialty shapes into the edges or interior of a sheet, forming multiple sheets into bound signatures or booklets, binding individual pages or signatures into books, and fastening covers to books by e.g. stapling, saddle-stitching, or gluing. Signature production requires folding a large printed sheet and cutting the folded stack so that the resulting cut pages are in sequential order.
When producing a booklet by folding sheets and nesting them together, toner applied to the fold area of a sheet can crack, reducing image quality along the fold. This can be particularly noticeable on the cover of a booklet.
Numerous approaches to reducing toner cracking have been proposed. For example, Japanese publication no. 2007-084324 to Morita describes heating toner in the fold area while folding. This requires an additional heater, and can reduce image quality if warm, partially-liquid toner runs or contacts other toner or parts of the machine. U.S. Publication No. 2008/0166647 to Mang et al. describes a toner formulated to reduce cracking, and Wales describes XTREME COATED COVER paper by MILLMAR PAPER, which includes a laminated coating to reduce cracking (Wales, Trish. “Paper reinvented.” Graphic Arts Monthly March 2010: 16-19, esp. pg. 18). However, it is desirable to permit use of a wide variety of toners and papers in a printer.
WO 2008/051943 to Jacobs et al. describes a system for detecting problems resulting from the interaction of toner and finishing system and providing a user the choice of alternative finishing methods. However, full-bleed covers (for example) must be printed across fold lines, so no alternative finishing methods exist.
U.S. Publication No. 2008/0252062 to Kelley describes a method for scoring one or more sheets in a booklet at two different and parallel locations to reduce the stress on toner at a fold line. However, this scheme can only be applied to double-creased booklets, which approximate the look of a perfect-bound book. It is desirable to produce booklets of various spine shapes.
Japanese Publication No. 2006-209427 to Sugita describes a system for reducing density of an image in a fold area. However, Sugita reduces density, after ripping (para. 37-38) but before screening or halftoning (paras. 44, 49), by limiting the total amount of toner applied per unit area to less than a selected maximum toner total amount (paras. 16, 24, 40) by multiplying the ripped gray levels with the toner limit (paras. 42, 48). This can result in highly-visible color shifts and other objectionable visual differences between the fold area and the non-fold area.
Commonly-assigned U.S. Publication No. 2008/0159786 to Tombs et al., the disclosure of which is incorporated herein by reference, describes printing raised information with a distinct tactile feel using electrophotographic techniques. Toner stack heights of at least 20 μm are provided. As toner stack height increases, the probability of toner cracking along fold lines also increases.
There is a continuing need, therefore, for a way of reducing toner cracking in fold areas without producing objectionable artifacts.